For practice or for a practice

Hi. Please tell me which phrase to use for each situation, "for a practice" or "for practice." Let's say you're playing a short class spelling game, where the first word for the students to spell is "delay," but it is for a practice (for practice?). Let's also say it is April Fool's Day and you give your students a worksheet on arithmetics and later you say, "Sorry (Ha ha), it was for a practice (for practice?).